Many large vehicles use air brake systems. These brake systems include air actuated service brakes coupled to service brake actuators. Pressurized air, typically at 100 psi, is applied to the service brake chambers of the service brake actuators to apply the service brakes. To keep the brakes applied when parked, combination brake actuators are usually used. A combination brake actuator includes a spring brake portion and a service brake portion. The spring and service brake portions include respective spring and service brake chambers, each defined in part by a piston or, more commonly, by a diaphragm assembly, connected to a push rod assembly. The push rod assembly is connected to the brake, typically by a slack adjuster. The spring brake portion also includes a heavy actuator spring, coupled to the push rod assembly, which tends to apply the brake. Supplying pressurized air to the service brake chamber applies the associated brake while supplying pressurized air to the spring brake chamber compresses the actuator spring to release the brake. Thus, when parked, air is exhausted from the service brake chamber which allows the actuator spring to apply the brake according to the force of the actuator spring.
One of the problems with conventional air brake systems is that the braking force generated by the spring brake portion of the combination brake actuator is only about 50% of the maximum braking force generated by an applied service brake. Therefore, with the spring brakes applied each axle with combination brake actuators has only about half the braking force which is available with the service brakes. One reason conventional combination brake actuators are so designed is to protect the brakes. That is, if the vehicle is parked and drum brakes are set while the drums are warm, upon cooling the drums have a tendency to contract which can, if the braking force is too high, result in damage to the brakes. Also, not all of the axles have combination brake actuators; due to economy and space requirements, often no more than half the axles are so equipped. Although the resulting braking force is sufficient for parking purposes, in an emergency, such as when service brake air pressure is lost, the braking force available is woefully inadequate.
Smaller automotive vehicles typically use hydraulic brake systems. Parking brakes with conventional hydraulic brake systems rely on a manual parking brake which is actuated by stepping on a pedal or pulling on a handle. The pedal or handle is connected to a cable which in turn pulls on a pair of cables ultimately connected to the rear brakes. While manually applied parking brakes ar generally adequate for most parking situations, they are quite limited in their ability to act as emergency brakes due to the low force exerted, the application of parking brakes to only the rear wheels and the lack of any sort of effective modulation of the braking force.